Distribution of Micro-Organisms Along a Transect in the South-East Pacific
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Biogeosciences, 5, 311–321, 2008 www.biogeosciences.net/5/311/2008/ Biogeosciences © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. Distribution of micro-organisms along a transect in the South-East Pacific Ocean (BIOSOPE cruise) using epifluorescence microscopy S. Masquelier and D. Vaulot Station Biologique de Roscoff, UMR 7144, CNRS et Universite´ Pierre et Marie Curie, Place G. Tessier, 29682, Roscoff, France Received: 6 July 2007 – Published in Biogeosciences Discuss.: 7 August 2007 Revised: 18 January 2008 – Accepted: 1 February 2008 – Published: 4 March 2008 Abstract. The distribution of selected groups of micro- 1 Introduction organisms was analyzed along a South-East Pacific Ocean transect sampled during the BIOSOPE cruise in 2004. The Unicellular picoplanktonic prokaryotes and eukaryotes less transect could be divided into four regions of contrasted than 2 µm in size (Sieburth et al., 1978) are found in marine trophic status: a High Nutrient Low Chlorophyll (HNLC) ecosystems at concentrations ranging from 102 to 105 and region (mesotrophic) near the equator, the South-East Pacific 102 to 104 cell mL−1, respectively. They play a fundamental Ocean gyre (hyper-oligotrophic), a transition region between role (Azam et al., 1983; Sherr and Sherr, 2000), in particular, the gyre and the coast of South America (moderately olig- in oligotrophic waters (Hagstrom¨ et al., 1988; Maran˜on´ et al., otrophic), and the Chile upwelling (eutrophic). The abun- 2001) where their small size associated to the reduced diffu- dance of phycoerythrin containing picocyanobacteria (PE sion boundary layer and large surface area per unit volume picocyanobacteria), autotrophic and heterotrophic eukary- are an advantage to acquire nutrients (Raven, 1998). The otes (classified into different size ranges), dinoflagellates, photosynthetic component of picoplankton, i.e. Prochloro- and ciliates was determined by epifluorescence microscopy coccus and Synechococcus cyanobacteria and picoeukaryotic after DAPI staining. Despite some apparent loss of cells algae, are important contributors to the microbial community due to sample storage, distribution patterns were broadly of the euphotic zone in many marine environments (Mackey similar to those obtained by flow cytometry for PE pico- et al., 2002; Perez´ et al., 2006). Heterotrophic protists play cyanobacteria and picoeukaryotes. All populations reached a pivotal role in mediating organic flux to higher trophic a maximum in the Chile upwelling and a minimum near the levels in pelagic ecosystems (Azam et al., 1983; Fenchel, centre of the gyre. The maximum abundance of PE pic- 1982; Hagstrom¨ et al., 1988). Among the heterotrophic pro- 3 −1 ocyanobacteria was 70 10 cell mL . Abundance of au- tists, ciliates and dinoflagellates are important grazers of pi- 3 totrophic eukaryotes and dinoflagellates reached 24.5 10 coplankton (Christaki et al., 2002). −1 and 20 cell mL , respectively. We observed a shift in the In the Pacific Ocean, picoplankton has been analyzed both size distribution of autotrophic eukaryotes from 2–5 µm in in the Equatorial region and the North gyre (e.g. Camp- eutrophic and mesotrophic regions to less than 2 µm in the bell et al., 1997; Mackey et al., 2002) but not in the South central region. The contribution of autotrophic eukaryotes to gyre. The latter is the most oligotrophic environment of the total eukaryotes was the lowest in the central gyre. Maxi- −1 world oceans based on SeaWifs imagery which provides esti- mum concentration of ciliates (18 cell mL ) also occurred mates of average surface chlorophyll a concentrations down in the Chile upwelling, but, in contrast to the other groups, to 0.02 mg m−3 (Morel et al., 2007). The BIOSOPE (Bio- their abundance was very low in the HNLC zone and near geochemistry and Optics South Pacific Experiment) cruise the Marquesas Islands. Two key findings of this work that explored this region sailing from the Marquesas Islands to could not have been observed with other techniques are the the coast of Chile. Along this transect, a gradient in trophic high percentage of PE picocyanobacteria forming colonies in conditions was encountered, from hyper-oligotrophic (gyre) the HLNC region and the observation of numerous dinoflag- to very eutrophic waters (Chile upwelling). The present ellates with bright green autofluorescence. study relied on epifluorescence microscopy to assess the dis- tribution in this region of phycoerythrin containing pico- cyanobacteria (called PE picocyanobacteria throughout the Correspondence to: D. Vaulot paper), autotrophic and heterotrophic eukaryotes (in par- ([email protected]) ticular dinoflagellates and ciliates). In contrast to faster Published by Copernicus Publications on behalf of the European Geosciences Union. 312 S. Masquelier and D. Vaulot: Micro-organisms in the South-East Pacific STB1 STB3 STB4 STB6 STB8 GYR2 STB11 STB14 STB17 STB20 Fig. 1. Map of the BIOSOPE cruise track superimposed on a SeaWiFS ocean colour composite, dark purple indicating extremely low values (0.018 mg m−3) of total chlorophyll a. Figure modified from Claustre et al. (2007). Stations analyzed by DAPI staining are labelled. Table 1. Concentrations of the different populations enumerated in the present study. Values are averages for the six depths sampled at each station. Picocyanobacteria containing Total Autotrophic Heterotrophic Total Autotrophic Heterotrophic Green Total phycoerythrin eukaryotes eukaryotes eukaryotes dinoflagellates dinoflagellates dinoflagellates dinoflagellates ciliates Station Latitude-Longitude mL−1 mL−1 mL−1 mL−1 mL−1 mL−1 mL−1 mL−1 mL−1 MAR1 08◦23 S–141◦14 W 3486 1520 1292 228 105 56 48 4.6 <1.5 HLN1 09◦00 S–136◦51 W 2818 2312 1836 476 93 61 32 4.2 3 STB1 11◦44 S–134◦06 W 1612 1895 1165 730 111 62 50 4.5 1.5 STB3 15◦00 S–129◦55 W 413 1423 737 686 59 28 31 4.2 3.5 STB4 17◦13 S–127◦58 W 374 1267 736 531 57 26 32 7.0 1.5 STB6 20◦26 S–122◦54 W 6 1413 726 687 37 19 17 2.2 1.5 STB8 23◦32 S–117◦52 W 37 937 521 416 31 12 19 3.5 4.5 GYR2 25◦58 S–114◦00 W 46 806 541 265 43 21 22 3.5 1.5 STB11 27◦45 S–107◦16 W 34 1050 526 525 31 10 21 6.5 <1.5 STB14 30◦02 S–98◦23 W 142 1314 854 460 55 22 33 8.5 4.2 EGY2 31◦50 S–91◦27 W 1734 3083 2481 602 82 47 35 6.5 1.9 STB17 32◦23 S–86◦47 W 1104 2607 2086 521 94 46 48 12 5.2 STB20 33◦21 S–78◦06 W 10 726 1760 1195 566 92 44 48 6.7 3 UPW1 34◦01 S–73◦21 W 40 548 3396 2526 870 122 63 59 15 10 UPX2 34◦37 S–72◦27 W 18 548 14 088 12 211 1877 151 47 104 38 6.6 enumeration techniques such as flow cytometry, epifluores- tigated extended from the Marquesas Islands (South Pacific cence microscopy allows (1) to discriminate specific group of Tropical Waters; SPTW) to the coast of Chile, through the organisms such as dinoflagellates, (2) to recognize cell orga- Eastern South Pacific Central Waters (ESPCW) which in- nization such as colonies, and (3) to regroup organisms into clude the centre of the Pacific gyre (Claustre et al., 2008). size classes. We attempted to relate the distribution of the The transect can be divided into four contrasted trophic different types of organisms to oceanographic conditions. zones (from West to East): a High Nutrient Low Chlorophyll (HNLC) zone (mesotrophic) near the equator, the South-East Pacific gyre (hyper-oligotrophic) proper, the transition zone 2 Material and methods between the gyre and the coastal region (moderately olig- otrophic), and the Chile upwelling (very eutrophic). In the 2.1 Oceanographic context hyper oligotrophic zone, nitrate concentrations were nearly undetectable between the surface and 150–200 m and re- The BIOSOPE cruise took place on board the French NO mained very low (∼2.5 µM) below this depth (Fig. 2 in “l’Atalante” in the South-East Pacific Ocean from 26th Oc- Raimbault et al., 2007). Nitrate concentrations were higher tober to 11th December 2004 (Fig. 1). The transect inves- in the HNLC zone and maximum in the Chile upwelling Biogeosciences, 5, 311–321, 2008 www.biogeosciences.net/5/311/2008/ S. Masquelier and D. Vaulot: Micro-organisms in the South-East Pacific 313 aa b5 µm c Fig. 3. Heterotrophic (a), autotrophic (b), and green fluorescing a dinoflagellates (c) observed under blue light excitation (top) and UV light excitation (bottom). Pictures taken at stations STB3 (20 m), UPW and STB7 (5 m), respectively. system equipped with 12 L Niskin bottles. In general, two samples were collected in the surface layer, three around the chlorophyll maximum and one below. Water was pre-filtered through a 200 µm mesh to remove zooplankton, large phyto- plankton, and particles before further filtrations. Water samples (100 mL) were fixed with glutaraldehyde (0.25% final concentration) and filtered through 0.8 µm pore c b size filters. This porosity was selected to avoid high densities of bacteria on the filter which would have rendered visuali- sation of the larger and less dense eukaryotes more difficult. Samples were stained with 4’6-diamidino-2-phenylindole (DAPI, 5 µg mL−1 final concentration) (Porter and Feig, 1980) and stored at –20◦C for a minimum of 12 months be- fore counting. Counts were performed with an Olympus BX51 epifluorescence microscope (Olympus Optical CO, Tokyo, Japan) equipped with a mercury light source and an x100 UVFL objective. Pictures of dinoflagellates were taken on board the ship on the freshly prepared slides using a BH2 Olympus microscope with an x40 objective and a Canon G5 digital camera.